Stealth-dicing is gaining popularity for singulation of wafers based on laser-induced crystal damage. The damage zones in the crystal define where the wafer separates into single dies when mechanical force/stretching is applied. Stealth-dicing offers high throughput and increases die count per wafer because dicing streets can be much narrower than with conventional blade-based sawing. For stealth-dicing to work properly, the backside of the wafer needs to be smooth to prevent optical scattering.
As used herein, the terms “front,” “front side,” “frontside,” and “top” refer to the side of the wafer upon which the BAW devices are built during the wafer process used to construct the circuits on a semiconductor chip. The terms “back,” “back side,” “backside,” and “bottom” refer to the side of the wafer opposite the front side.
For Bulk Acoustic Wave Solidly Mounted Resonators (BAW-SMR) devices, SAW devices, and Temperature Compensated SAW (TC-SAW) devices, having a smooth wafer backside is a liability. During operation of these devices, a small fraction ( 1/1000 to 1/10,000) of acoustic (wave) energy penetrates into the substrate material. If the wave bounces back from the backside of the substrate, a vertical standing wave will result. Lambda (λ) is the acoustic wavelength. At frequencies where the substrate thickness equals a integer multiple of λ/2, a noticeable disturbance and/or ripple in the electrical characteristics of the filter will occur. In typical BAW devices with a 110 micrometers (μm) thick silicon (Si) substrate (after backgrinding), there will be a ripple related to backside reflections occurring approximately every 37 MHz. For a filter operating at 2.5 GHz, the acoustic wavelength in Si is about 3.3 μm. Acoustic wavelength in SAW and TC-SAW at 1 GHz is in the range of 3 μm to 8 μm depending on the wave type leaking out below the resonator.
One conventional technique to prevent the creation of the standing wave is to deposit an impedance matching layer on the backside of the wafer. However, the impedance matching layer will introduce a frequency dependent backside reflectivity. This may work well for bandpass filters, but for notch filters this approach is useless as it does not prevent backside reflections at low frequencies. Also, it requires additional effort and expense to deposit a well-defined layer after the back-grinding process.
Another conventional technique to prevent the creation of the standing wave is to roughen the backside of the wafer substrate using mechanical and/or chemical means, and thus disperse the reflected wave at different angles. However, after conventional roughening techniques, BAW wafers are too rough for stealth-dicing, because of the optical scattering of the roughened backside. Access to the saw streets from the frontside is not possible because grounding connections and Process Control Monitor (PCM) structures occupy the dicing streets.
Thus, stealth-dicing cannot be performed after the wafer backside has been subject to conventional roughening using mechanical and chemical means. Nor can stealth-dicing be performed before the wafer backside has been subject to conventional roughening using mechanical and chemical means: after the stealth-dicing step, the wafer is fragile and may break apart during the
Therefore, there is a need for stealth-dicing-compatible devices and methods to prevent acoustic backside reflections on acoustic wave devices.